Digital printing is an effective method of image transfer which is currently getting popular as it automates and streamlines the printed image without much operator intervention. Image is formed on the imaging surface by use of a uniform electrostatic charge distribution throughout the surface. The electrostatic charge can be created by the corona and the print image is developed by applying toner which adhere to the surface and develop the image. Common digital printing presses available in market are from HP indigo, Kodak, Xerox, Canon, Ricoh Konica etc.
Digital printers usually use either liquid or dry toners for printing. Liquid ink is used by HP indigo. HP indigo follows offset process where ink is transferred from photo imaging cylinder to offset cylinder and then to the substrate. Dry toner based ink is used by Konica Minolta and Xerox Digital. Konika Minolta talks about using low temperature fusion Simitri HC toners to give a offset finish where fixing temperature requires to be as low as 150° C. as mentioned in their U.S. Pat. No. 8,304,157 and Xerox uses low melt EA (Emulsion Aggregrates) dry ink technology which also fuses with substrate at low temperature.
It is well known that electrostatic properties depend upon the nature of the substrate. For any machine working by electrostatic function, electrostatic properties are of atmost importance. It is well known fact if any substrate comes in contact with any other substrate or material coefficient of friction, static properties etc art important to determine the easy movability of the substrate. These properties can be easily determined by simple testing instruments.
Substrates to be printed are fed into the printing press in either sheet form or in roll form. In sheet form, the sheets are put in a tray and sheets either slide or are picked one by one or any other method known in the art and sent to the printing section for printing. For easy sliding and picking, minimization of static charge is of foremost importance. If the substrate has high static, it will not slide easily or be picked up easily or multiple sheet will be picked or slide together. In roll form, the substrate from the unwinder moves to printer and after printing rolls move to the winder roll.
The substrate is later further processed as per requirement.
The U.S. Pat. No. 7,279,513 describes coatings for paper consisting of Ethylene Acrylic Acid (EAA) for digital printing. This patent specifically discloses EAA coating over paper only and fails to disclose combination of EAA and vinyl acetates or acrylates coatings be applied over the oriented polyester film. A drawback of using EAA coating over film is of static charge. EAA has high static charge hence, cannot be used in all types of digital printing processes due to poor runability. EAA Coated films are coated by off-line method and EAA coating has been mentioned only for HP Indigo printers which use liquid toners.
It is an object of the present invention to overcome the difficulties presently encountered in the art of providing the coated oriented polyester film which is capable of receiving both liquid as well as dry toners on all types of the printers.
The inventive film exhibits properties that are significantly different from those of another coated polyester film where the film is coated by acrylic, copolyester or polyurethanes. The other coated polyester films are printed by using engraved cylinders. The inventive film is used for electrostatic printing. In the electrostatic printing process, the printing is done by means of electrostatic charge. The present invention relates to an oriented polyester film, having electrostatic printing characteristics and is capable of printing on all types of electrostatic printers with liquid and dry toners such as Xerox iGen digital printing press, digital indigo HP ink systems, Ricoh Digital Press, Canon digital press, Kodak Digital press etc.